The present invention relates to a method of controlling the envelope of a radio signal to be transmitted from a station for mobile radio communications, and apparatus for implementing the method.
The technical field that is targeted more particularly by the invention is that of stations for mobile radio communications using a digital cellular system having time-division multiple access, of the Global System for Mobile communications (GSM) type, of the Digital Cellular System (DCS) type, or of the Personal Communication System (PCS) type.
In such a system, the various stations, i.e. firstly the base stations, and secondly the mobile stations, such as terminals on board vehicles, and portable and handheld cellphones, interchange data which is transmitted in the form of packets or bursts. Each burst is transmitted by a station in a predetermined time slot of a given frame structure. Each burst further has a particular shape for the envelope of the radio signal, which shape presents a plateau corresponding to the payload portion of the signal to be transmitted,.e.g. the speech in code form, the plateau being preceded by a rising ramp and followed by a falling ramp corresponding to guard times which surround the payload portion of the signal.
Conventionally, a burst is formed at the transmitter of a station by using a power amplifier to amplify the power of the radio signal to be transmitted, and by controlling the envelope of the radio signal output by the power amplifier, control being achieved by applying a voltage that is variable as a function of the desired envelope shape to a control input of the amplifier. The power amplifier conventionally uses one or more field effect transistors. For example, the amplifier may be made using Metal Semiconductor Field Effect Transistors (MESFETs), or High Electron Mobility Transistors (HEMTs) or Pseudomorphic HEMTs (P-HEMTs), on an AsGa or InAsGa substrate.
FIG. 1 is a simplified diagram showing known apparatus for controlling the envelope of a radio signal for the purposes of forming a burst of given envelope, using the above-described principle.
With reference to this figure, a radio signal ERF coming from an input circuit (not shown) is amplified by means of a power amplifier 1, e.g. a power amplifier of the above-described type. A fraction of the power of the signal output by the power amplifier 1 is tapped by means of a directional coupler 2, and it is delivered to a servo-control loop including an envelope detector 3. Comparator means 4 for comparing a first signal V1 as output by the envelope detector 3 with a reference second signal VREF as output by a ramp generator 5 and which is a function of the desired envelope. The signal Sc output by the comparator means 4 in turn controls control means 6 which modify the control voltage Vdd applied to the power amplifier 1 as a function of the result of the comparison.
The above-described control loop thus makes it possible to generate a burst SRF that has the desired envelope and that is applied to an output circuit (not shown) including a transmit antenna.
Naturally, the ramp generator 5 may be replaced by any other generator for generating a reference signal corresponding to the desired envelope, and making it possible to control the power level of a radio signal output by a power amplifier so that it corresponds to said envelope.
The drawback of the apparatus shown in FIG. 1 lies in the envelope detection principle itself.
The directional coupler 2 required to tap the radio signal at the output of the power amplifier 1 so as to detect its envelope gives rise to insertion losses after the amplifier 1. Thus, in order for the maximum power of the signal SRF to correspond to the requirements set by standards (typically 2 watts for a handheld GSM terminal, and 1 watt for a handheld DCS or PCS terminal, the signal output by the power amplifier must be of power higher than the power of the signal SRF, in order to take into account the insertion losses. As a result, energy consumption is not optimized in a field in which a constant concern is to reduce energy consumption to as little as possible, in particular in battery-operated handheld terminals.
Furthermore, the station for mobile radio communications must be capable of transmitting at power levels lower than the maximum level set by standards. Thus, the envelope detector must have a broad dynamic range in order to operate correctly both at maximum power and at minimum power.
An object of the invention is to provide a method and apparatus for controlling the envelope of a radio signal without it being necessary to detect the envelope of said signal.
To this end, the invention provides a method of controlling the envelope of a radio signal to be transmitted from a station for mobile radio communications, said radio signal being output by a field effect power amplifier, said method comprising the following steps:
tapping a control voltage applied to the power amplifier;
comparing the tapped control voltage with a predefined reference voltage; and
causing said control voltage applied to the power amplifier to vary as a function of the result of the comparison.
The present invention further provides apparatus for controlling the envelope of a radio signal to be transmitted from a station for mobile radio communications, said radio signal being output by a field effect power amplifier , said apparatus comprising:
means for tapping a control voltage applied to the power amplifier;
comparator means for comparing the tapped control voltage with a predefined reference voltage; and
control means for causing said control voltage applied to the power amplifier to vary as a function of the result of the comparison.
The invention is the result of tests conducted by the Applicant and showing that, in the particular case of a field effect power amplifier implemented using transistors on a substrate based on AsGa, the power of the signal output by such an amplifier is a faithful image of the control voltage that is applied to it and that determines the drain bias of the amplifier AsGa transistor(s) making up the power amplifier. The invention is applicable to amplifiers of the above-described type because they are very temperature stable, and they therefore deliver the same output power for a given control voltage, regardless of temperature. Thus, it is possible to control the level of the power of the signal output by the amplifier without having to tap the signal and without having to detect its envelope. It is necessary merely to tap the amplifier control voltage and to servo-control the value of said voltage as a function of the result of the comparison between said voltage and the reference signal.